Fertility and clonality of Pickeringia (Fabaceae), a unique endemic genus of the California chaparral.

The evergreen, spiny shrub pea Pickeringia (Fig. 1) is a showy, occasional component of chaparral and adjacent vegetation communities across the Coast, Transverse, and Peninsular ranges of California—from Mendocino County to northernmost Baja California. It also survives in a handful of outlying colonies in the Sierra Nevada foothills and the northern Channel Islands (Fig. 2). An outstanding phylogenetic analysis by Wojciechowski et al. (2004) clarified its relationships within Sophoreae, where it forms a three-part clade with the temperate-to-subtropical genera Cladrastis and Styphnolobium. The latter consists of about eight species that occur in the warmer parts of North and Central America and one in China. Cladrastis, in contrast, has about eight species in eastern Asia and one, C. kentuckea, in warm temperate North America. This trio occupies an unusually isolated branch of the legume family tree and, within it, Pickeringia is estimated to have diverged during the Oligocene, roughly 31 million years ago (Wojciechowski, 2013)—marking it as a true relict in the modern vegetation of California and Baja California. In the pages that follow, we explore the history, ecology, and evolutionary distinctiveness of this remarkable genus. 

The clade that includes Pickeringia was once broadly distributed in the temperate to subtropical vegetation of the Northern Hemisphere in the late Paleogene (Wojciechowski, 2013). As the current summer-dry, mediterranean-like vegetation began to emerge from the middle Miocene onward, genera capable of surviving under these new climatic and ecological conditions—like Pickeringia—formed the chaparral and related vegetation types that have spread to dominate wide areas regionally until the present. 

Botanists have long held that Pickeringia only rarely reproduces by seed, relying instead primarily on vegetative sprouting from spreading roots and underground stems—a view summarized by Wojciechowski (2013). As we will show, however, the apparent sterility applies to some, but far from all, of its populations. This pattern raises broader questions about how infertility evolves in clonal or relictual lineages, as explored further below.